An ATPL is not a CPL/IR! Current modular and intergrated schemes train you to CPL flying standards with IR and ATPL ground subjects (frozen ATPL). In the old days you would complete a CPL flying course, an IR flying course and then sit the CPL ground subjects. When you reached the minimum flying hours/experience requirements for the ATPL you would sit the ATPL ground exams. Later the CAA decided that you could sit the ATPL ground subjects before having the required hours for the ATPL and the FROZEN ATPL was born.
The MPL will be the equivalent of a CPL/IR with ATPL ground subjects i.e. a Frozen ATPL. Once you reach the minimum Flying experience standards you will be able to apply for the full ATPL in just the same way that you would have done if you were unfreezing your frozen ATPL on the old schemes.The only difference being that once you complete an MPL course you will only be allowed to fly the type of commercial aircraft you were trained on but as you will complete this kind of course with a reasonable chance of a job with the company that 'sponsors' you. Once you are issued with your ATPL the world is your oyster!
PS The requirements for unfreezing are approz 1500 hours experience, so no outside training would help, that said it is an experience requirement mostly so some single piston flying etc would help, check the Lasors/CAA website for full requirements.